Mental retardation is a prominent clinical feature of the lysosomal storage disorders (LSD's), due to the severe pathology in the central nervous system (CNS). The LSD's constitute a significant number of the human genetic diseases that affect the CNS. The LSD's are usually not diagnosed until significant pathology has already occurred, typically in the first year of life. Storage lesions and secondary pathology are widespread in the brain. Thus, treatment needs to arrest or reverse the disease in substantial volumes of brain tissue and it should be life-long. Vector-mediated gene transfer into the CNS can mediate long-term, sustained levels of gene expression in experimental animals. However, no gene transfer method so far devised can achieve both global gene transfer and stable high-level gene expression at the stage of brain development when these patients need to be treated. Our hypothesis, based on demonstrated results from this grant, is that effective gene therapy can be achieved instead by delivering the enzyme widely in the brain. The strategy is based on the proven principle of cross-correction in which genetically modified cells release normal enzyme that is utilized by neighboring and distal cells. Thus the goal of the grant is to develop methods to increase the spatial distribution of enzyme through better understanding of the properties of AAV vectors in the mammalian brain. Animal models of a human LSD, mucopolysaccharidosis (MRS) type VII, will be studied. Some experiments will need to be done in the MRS VII mouse, but the most promising improved methods will be tested in the cat brain, which is 100 times larger than the mouse's and is more similar in structure to the human brain, as a translational model. MRS VII is the best model for conducting these studies because the enzyme can be detected in situ with a very sensitive histochemical reaction for the biologically relevant enzymatic activity, which provides an unparalleled experimental system to follow the fate of gene transfer and enzyme spread in three dimensions in the CNS. We will: 1) evaluate a new vector which significantly increases total gene expression and enzyme secretion;2) study the differential distribution of enzyme and vector genome transport via axonal pathway to ascertain whether targeting specific structures can deliver the enzyme more widely;3) evaluate the ability of a chimeric recombinant protein to facilitate delivery to the enzyme, including from peripheral sites of inoculation into the CNS;and 4) evaluate the effects of increased delivery on disease correction, including storage lesions, neurodegeneration, and abnormal neurophysiology. Although progress towards these goals has been made in the mouse brain in the last 5 years, to achieve the magnitude of scale up needed for a large mammalian brains will require better understanding of the properties of AAV gene transfer and enzyme fate in the CNS.